During the latent phase of tuberculosis the tubercle bacillus is believed to be metabolically active but in a slow or non-growing state which can resume bacterial replication at an opportune time later in life. However the mechanisms involved in the cessation of Mycobacterium tuberculosis (Mtb) division are unknown. There have been recent descriptions of bacterial possessing systems, resembling plasmid encoded toxin-antitoxin proteins that induce cell growth arrest when the microorganism is exposed to an environmental stress. We and other investigators have identified proteins resembling these putative toxin-antitoxin modules in Mtb. To provide evidence that these systems are cell growth arrest modules involved in the persistent phase of Mtb infection, we propose: 1-To demonstrate the inhibitory effect of the putative Mtb "toxin-antitoxin modules" on bacterial growth in vitro. 2-To determine the patterns of expression of the putative Mtb "toxin-antitoxin modules" during the persistent phase of bacterial infection in mice lung and guinea pig granulomas. 3-To analyze the regulatory mechanisms for the expression of the putative Mtb "toxin-antitoxin modules". 4-To determine the mechanism of action of the putative Mtb '_toxin-antitoxin modules". 5-To evaluate the role of the Mtb "toxin-antitoxin modules" in bacterial virulence, using a model of mouse lung infection. These studies will provide insight into the latent phase of tuberculosis and they will allow us to identify possible targets form the design of antimycobacterial drugs. During the research supported by this proposal the candidate will gain knowledge in the study of transcriptional regulation of Mtb under the mentorship of Dr. Issar Smith, who is an expert in the field. The candidate will obtain expertise in the management of State-of-the-Art techniques at the Public Health Research Institute, an Institution with a high level of scientific achievement in the area of infectious diseases like TB. The completion of this proposal in this framework of scientific and institutional support will be fundamental for the candidate to establish herself as an independent investigator in the field of Mtb pathogenesis.